Abstract
Aluminum ions interfere with the determination of fluoride by the alizarin complexon(ALC)-extraction method. A method is described for eliminating the interference, in which fluoride ions are liberated by co-precipitating aluminum ions with iron(III) hydroxide at pH 10. Fluoride ions are then extracted into an organic solvent and determined by the ALC method. This method was easier than others, and reliable results were obtained.
The recommended procedure is as follows. A sample solution containing 5 to 50 μg of fluoride and less than 250 μg of aluminum ions is prepared and made up to about 50 ml with water. Five milliliters of 0.1 M iron (III) chloride solution are added and pH is adjusted to about 10 by the addition of 2 M ammonium hydroxide. The solution is boiled for a few minutes, and filtered through a filter paper. Precipitates are washed thoroughly with warm water. The filtrate and washings are combined, and the pH is adjusted to 57 with dilute hydrochloric acid. The solution is transferred to a 100 ml volumetric flask and made up to the volume with water.
A 20 ml portion of the solution is transferred to a 100 ml separating funnel, and 20 ml of lanthanum-ALC reagent solution {prepared by mixing 75 ml of acetate buffer solution(pH 4.4), 25 ml of 0.003 M ALC solution, 200 ml of acetone, 25 ml of 0.003 M lanthanum chloride solution, and about 180 ml of water} is added.
After allowing the solution to stand for about 10 min, 10 ml of N, N-diethylaniline in iso-amyl alcohol (volume fractions≈5%) is added, and the fluorine compound is extracted into the alcohol by shaking for 3 min, and the two layers are allowed to separate by standing. After the drainage of the aqueous layer, the organic layer is washed with about 20 ml of acetate buffer (pH 4.4) solution.
The absorbance of the organic layer is measured at 580 nm using a reagent blank as reference.
By this method, 2550 μg of fluoride ions in the synthetic samples which contained less than 250 μg of aluminum was determined with 94% recovery.
